1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode power electronic converter circuits. The subject invention is a continuation of a pending patent whose Ser. No. is 09/956,711, filed Sep. 19, 2001.
2. Description of Related Art
For power converter applications that require large step down ratios, such as providing power to a microprocessor or DSP core, there are several possible solutions. In some cases an isolated converter form such as a forward converter is chosen so that the transformer can be used to provide the voltage step down and current step up. Compared to a simple buck converter the forward converter. or any other isolated converter type requires considerably more silicon for the same power losses. It is well known to those who are skilled in the art of power converter design that the component load factor for any isolated converter is four times that of a simple buck converter. The simple buck converter, or a synchronous rectifier buck converter, is often a better choice despite the large step down ratio issue. When a buck converter is chosen for a high step down ratio application the upper mosfet is chosen to be smaller than the lower mosfet since the duty cycle for the upper mosfet is small and the duty, cycle for the lower mosfet is large. Also the upper mosfet is chosen to have low gate drain capacitance so that it can switch fast to avoid the large turn on switching losses. Another option is a tapped inductor buck converter, as shown in FIG. 1. An alternate arrangement of the tapped inductor buck converter is shown in FIG. 2. The tapped inductor buck converter has the same low component load factor as the simple buck converter, but it, in effect, provides the step down ratio of a transformer isolated topology. The tapped inductor changes the buck converter in a couple of ways. The upper mosfet now sees a higher applied voltage and lower current and the lower mosfet now sees a lower applied voltage and higher current. The duty cycle of the lower mosfet is decreased and the duty cycle of the upper mosfet is increased. The net effect is that the component load factor is the same as the buck converter. There is a simple reason why the tapped inductor converter is not more popular. In the buck converter the output current is non-pulsating or continuous, although the input current of the buck converter is pulsating. For the tapped inductor buck converter both the input current and the output current are pulsating. What is needed is a tapped inductor buck converter that can achieve non-pulsating currents at both input and output terminals and elimination of first order switching losses.
Most soft switching or zero voltage switching (ZVS) converters require a brief dead time between operation of the switches to achieve zero voltage switching. The amount of dead time required is dependent on the current magnitudes and component values so that the chosen dead time is often close to the amount of dead time needed for a range of currents but often the fixed dead time results in additional losses because the switch is turned on too soon when there is applied voltage to the switch or too late after the switch body diode or other parallel diode has been conducting for a time and dissipating power at a rate greater than the switch would dissipate if it were turned on at the best possible time. What is needed is a simple gate circuit that senses the mosfet drain voltage and enables the mosfet at the instant when the drain source voltage drops to zero.
One significant source of power losses in high frequency power converters is gate drive loss. Some converters have the inherent ability to provide synchronous rectifier self drive which results in the recirculation of gate drive energy. What is needed is more synchronous rectifier self drive mechanisms for more converter topologies.
Another source of power losses is current sensing. Often a resistor is place in series with a high current path to accomplish current sensing. Often the resistor is made small in value and high in power dissipation ability. If the resistor is too small the signal can be noisy and additional circuits are often required to amplify the signal. Alternately, a current transformer circuit can be used, but these are relatively large and expensive and still require a significant amount of power dissipation in the high current winding of the current transformer. A circuit mechanism that has low noise and is low in power dissipation or lossless is needed.
An object of the subject invention is to provide a ZVS cell that eliminates first order switching losses in all switches in a tapped inductor buck converter.
Another object of the subject invention is to provide a ZVS cell that also functions as an input and output filter to provide both continuous input terminal current and continuous output terminal current in a tapped inductor buck converter.
Another object of the subject invention is to provide a circuit mechanism that can accomplish optimal gate switching timing for switches that turn on at zero voltage.
Another object of the subject invention is to provide a simple self drive mechanism for the synchronous rectifier which recirculates rather than dissipates gate drive energy for the synchronous rectifier mosfet(s).
Another object of the subject invention is to provide a simple, reliable, and lossless current sense mechanism for high currents that does not require a resistor or transformer winding in the high current path.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.
These and other objects of the invention are provided by novel circuit techniques that add a small inductor, an auxiliary switch, and a capacitor to achieve ZVS and continuous terminal currents for the tapped inductor buck converter. Optimal gate timing is provided for N channel mosfets by a simple circuit consisting of a small P channel mosfet and two rectifiers. Synchronous rectifier self drive is provided by coupling a capacitor to the same small inductor used to provide input and output filtering and drive energy for ZVS. The same circuit used to provide ZVS can be used as a base to provide a signal that is an analog to the peak inductor current.